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Scientist who found '10th planet' discusses downgrading of Pluto

In this artist’s conception, Eris, which was recently ruled a dwarf planet, looms in front of its moon, Dysnomia, with the distant sun in the background.

Michael Brown

Still mourning the loss of Pluto as a planet? Blame Caltech astronomer Michael Brown. His 2005 discovery of Eris, a new "planet" past Pluto, prompted other astronomers to redefine what makes a planet, and eventually rule out Pluto and Eris and settle on eight true planets. Pluto and Eris now have the new designation of dwarf planet.

Brown will present the 25th annual Bunyan Lecture at Kresge Auditorium on Wednesday, Oct. 25, at 7:30 p.m., telling the story of his discovery and explaining why it prompted changing the definition of a planet. His talk, titled "Pluto, Eris and the Dwarf Planets of the Solar System," is free and open to the public. The Astronomy Program in the Department of Physics will host the event.

Although initially disappointed that his discovery, Eris, was demoted, Brown said, "It was the right thing to do. I think eight makes much more sense."

Brown scanned the sky for seven years before finally finding what he thought was the "10th planet." Eris is slightly larger than Pluto and orbits three times farther from the sun than Pluto—making Eris the most distant object ever seen revolving around the sun.

The discovery of Eris brought up a dilemma: Just what qualifies as a planet? The familiar idea of nine planets around the sun was dear to many. But for years astronomers had protested that the old classification system, based on purely historical considerations, just didn't make sense. With a new object requesting entry to the club, and the possibility of many more, astronomers had to rethink the issue.

The International Astronomical Union (IAU), a group of scientists that decides astronomical rules, convened a meeting in the summer of 2006 to debate the topic. Astronomers suggested several options, including one that would declare 53 planets currently exist in our solar system. In the end, the IAU defined a planet with three criteria. It must be large enough that its gravity makes it round, it must orbit the sun and it must have cleared its path of any other objects.

Because Pluto lies among ice, rocks and comets, it does not satisfy the path-clearing condition. Eris, for the same reason, also did not make the cut. That leaves eight planets, and little likelihood of more to come. Pluto and Eris were relegated to a new category of objects called dwarf planets, which fulfill the first two rules for planets but don't clear their paths and aren't moons.

Hoping to glimpse new planets, Brown and his team since 1995 have used telescopes at the ground-based Palomar Observatory in Southern California to systematically scan every inch of the sky. To date they have found about 80 objects in the distant regions of the solar system, most of them small.

"The sky is a big place," Brown said, so it took a long time for something as large as Eris to turn up.

For each patch of space they took three photographs over three hours, then used computers to compare the pictures and look for objects that move from one to the next. Because distant stars' positions stay stable in relation to each other, something that moves from frame to frame must be much closer, spinning through our own solar system.

The first time Brown's team members analyzed photos of Eris they missed it. Because Eris is so far away, it appears to move very slightly. Only by going back to the shots for a more thorough look did Brown and his team finally find Eris.

They nicknamed Eris "Xena" at first and called its moon "Gabrielle" after a duo from the TV show Xena: Warrior Princess. But in October 2006 the IAU approved the official name as Eris. When it came time to formally christen the dwarf planet, Brown chose a "good Greek" mythological name.

"Eris is the Greek goddess of discord and strife," Brown said. "She seems like the obvious choice after this whole argument about Pluto." Eris' moon is named Dysnomia, after the mythological daughter of Eris.

The planet debate and its ensuing hoopla has brought fame to Brown, who was one of Time magazine's 100 most influential people this year. Brown finds this ironic, however, because "I'm the third most influential person in my house."

The Bunyan Lecture honors the late James T. Bunyan, a member of the Hoover Institution, whose will specified that his estate endow lectures that "inquire into man's changing vision of the cosmos and of human destiny as revealed in the latest discoveries in the fields of astronomy and space exploration."